Final Part 1 Flashcards
Refrigeration (definition)
System allows transfer of heat from the cooling chamber to a location where heat can easily be discarded
Refrigeration Effect (definition)
The enthalpy difference of the refrigerant between the inlet and the outlet location of an evaporator
Refrigeration Effect (equation)
H2 - H1
Components of a refrigeration system
Evaporator
Compressor
Condenser
Expansion Valve
Evaporator
Liquid refrigerant vaporizes to a gaseous state (latent heat)
Compressor
Raises the pressure and temperature of the refrigerant
Condenser
transfers heat from the refrigerant to another medium (air/water) by rejecting heat (gaseous refrigerant condenses to liquid)
Expansion Valve
A metering device that controls the flow of liquid refrigerant to an evaporator
Compressor (equation)
q = m(H3 - H2)
Condenser (equation)
q = m(H3 - H1)
Evaporator (equation)
q = m(H2 - H1)
Coefficient of performance (COP)
= (H2 - H1)/ (H3 - H2)
Practical Storage Life (PSL)
the period of frozen storage after freezing during which the product retains its characteristic properties and remains suitable for consumption
High Quality Life (HQL)
the time elapsed from freezing of an initially high-quality product and the moment when a statistically significant difference from the initial can be established
Just noticeable difference (JND)
The observed difference from the high-quality life and the initially high-quality product
Direct Freezing
Product and refrigerant have direct contact
Indirect contact
Product and refrigerant are separated by a barrier throughout the process
Components of a microwave
Power Supply
Magnetron
Wave Guide
Stirrer
Oven Cavity
Power Supply Function
Draw electrical power and convert to high voltage required by magnetron
Magnetron Function
Converts power into microwave energy
Wave guide function
transfers the generated energy to the oven cavity
Stirrer Function
distributor that scatters the energy throughout the oven
Oven Cavity function
encloses food to be heated with the metallic walls
Which agency regulates microwaves
FDA
Overall heat transfer coefficient
measure of the rate of heat transfer through a composite system
Plate Heat Exchangers
- Indirect heating
Application: Beverages and dairy
Tubular Heat Exchanger
+ non-contact
+ Indirect
Steam Infusion Heat Transfer
+ Direct heat exchanger
Application: cooking, sterilization, pudding, etc.
Evaporators (key components)
+ Heat exchanger enclosed in vacuum chamber (product boils at low temp)
+ Condenser
Batch-Type Pan (Evaporator)
+ Spherical vessel
+Steam Jacked
+ Connect (vacuum)
Poor heat transfer
Natural circulation evaporators
+Short vertical tubes
product rises (heated) steam condenses on the outside of tube
+ Evaporation take place inside
+ Falls on base vessel through central annular section
Rising-Film Evaporators
+ Tubes heated from the outside as liquid vaporizes (film formed moves upwards)
+ High convective heat transfer coefficients
Falling-Film Evaporator
+ Thin film moves downwards
+ Viscous fluid
Advan: allow more effects
Best suited for heat sensitive (OJ)
Combined Rising/Falling Film
+Connected in tandem
+ Concentrated by circulating
* Through the rising-film section
* Followed by the falling-film section
Forced-circulation evaporators
+ Liquid is forced at high rate
+ /\T heating surface
+Comparative
- Low capital & operating cost
Modes of Heat Transfer
Conduction
Convection
Radiation
Specific Gravity (SG)
A measure of the density of a substance compared to the density of water
Absolute Pressure
the pressure in a space relative to a perfect vacuum, or zero pressure
Refractometer
Measures the amount of total soluble solids in a solution
U-tube manometer
measure pressure differences in various processing and storage systems
Dry bulb temperature
Temperature reading indicated by normal
thermometer reading or sensor
Wet bulb temperature
The thermometer or sensor covered with
wetted wick
Dew Point Temperature
Water vapor present in the air
(considered steam at low pressure)
Humidity ratio (W)
mass of water vapor per unit of dry air
(referred as moisture content/specific humidity)
Specific Volume
The volume of 1 kg dry air plus water vapor in
air (m³/kg)
Moisture Removal
Diffusion of liquid or vapor through the
product structure
Moisture removal process
Simultaneous heat and mass transfer
Drying-Rate Curve: A-B
initial removal of
water
Drying-Rate Curve: B-C
Constant Rate Period
+ Significant reduction
of moisture occurs
+occur with the
product at the wet-bulb
temperature of the air
Critical Moisture Content (B-C)
When an abrupt
change in the rate of
moisture removal
occurs
Drying-Rate Curve (C-D)
Falling Rate Period
+Moisture removal
begins to drop
significantly with time
Drying-Rate Curve (D-E)
More fall rate periods
+This behavior is
product dependent and
other factors such:
o Pre-treatment
o Water activity
o Drying conditions
